The ‘exceptionally sized’ diamond to be auctioned by Sotheby’s in London.
Photograph: Sotheby's/PA

A large diamond ring is expected to fetch £350,000 at auction 30 years after its owner paid £10 for it at a car boot sale, thinking it was a costume jewel.

The “exceptionally sized” stone was presumed not to be real because 19th-century diamonds were not cut to show off their brilliance like today’s gems. The owner, unaware of its value, wore it for decades, while doing everything from the shopping to the chores.

Sea lion grabs girl from dock and pulls her underwater

Read more

The 26.27-carat, cushion-shaped, white diamond, snapped up at a Sunday sale at the West Middlesex hospital in Isleworth in west London in the 1980s, is going under the hammer at Sotheby’s on 7 June.

The head of the auction house’s London jewellery department, Jessica Wyndham, said: “The owner would wear it out shopping, wear it day-to-day. It’s a good-looking ring. But it was bought as a costume jewel. No one had any idea it had any intrinsic value at all. They enjoyed it all this time.

“They had been to quite a few car boot sales over the years. But they don’t have any history of collecting antiques and they don’t have any history of collecting diamonds. This is a one-off windfall, an amazing find.”

Wyndham said that after about 30 years of wearing the ring, the owners brought it into Sotheby’s after a jeweller told them it could have substantial value.

“They came in with the idea that it might be real and they had no idea of its value,” she said. “We had a look and said … I think that’s a diamond, and we got it tested at the Gemological Institute of America. The majority of us can’t even begin to dream of owning a diamond that large.”

Wyndham said the owners, who did not want to be named, are “incredibly excited. Anyone would be in this position. It’s a life-changing amount of money. No matter what your background is or what your past experiences have been, it’s going to revolutionise someone’s life”.

The diamond is thought to have been cut in the 19th century but its history and how it arrived at the car boot sale are unknown.

Wyndham said the older style of diamond cutting was “slightly duller and deeper than you would see in a modern style … it could trick people into thinking it’s not a genuine stone”.

She added: “The diamond cutter is looking to maximise the amount of brilliance from the stone because that’s what makes it sparkly,” she said.

“With an old style of cutting, an antique cushion shape, the light doesn’t reflect back as much as it would from a modern stone cutting. Cutters worked more with the natural shape of the crystal, to conserve as much weight of the crystal rather than make it as brilliant as possible. The older stones have quite a bit of personality. They sparkle in a different way.”

“Serendipity” may have prompted the owners to buy the ring in the first place, she said, “but a 26-carat, cushion-shaped diamond, no matter how filthy or dark that mount was, maybe that stone still speaks to people. They also bought some designer dresses for about £10. We should have all been at the car sale!”